Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described, e.g., in Principles of Non-Impact Printing by Jerome L Johnson--Palatino Press--Irvine Calif., 92715 USA. Electrostatographic printing includes electrographic printing fn which an electrostatic charge is deposited image-wise on a dielectric recording member as well as electrostatically printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on said recording member.
The toner image is transferred onto a printing stock material, usually paper in the form of a web whereon the toner image is fixed, whereupon the web is cut into sheets containing the desired print frame. As can be learned from the book "The Physics and Technology of Xerographic Processes" by E. M. Williams (1984), Chapter Ten, p. 204 et seq the transfer of developed toner images onto paper proceeds by means of electrical corona devices to generate the required electric field to attract the charged toner from the electrostatographic recording member to the paper. The transfer efficiency of toner onto the receptor paper is not only dictated by the contact of the paper with the toner-laden recording member and the deposited charge but also by the conductivity of the paper and particularly by its water content. Paper is not a simple insulating dielectric, so the electrical properties of plain paper have some influence on toner transfer.
Experiments with a variety of paper types and thicknesses (i.e. weights) have established that heavier papers yield improvement in transfer efficiency. Paper types with high porosity, i.e. high permeability for gases loaded with ions by corona discharge do not allow an efficient toner transfer. Variation in gas permeability or porosity between different paper types is due to overall thickness, degree of filling with clays, sizings, and other paper treating substances.
Apart from the paper fibres and said substances which form a constant factor for conductivity or volume resistivity there is the moisture content which fluctuates with the humidity of the environment, especially the environment of the paper storage unit containing the paper on roll.
It has been established that as the moisture content increases from about 3 to 10% by weight, the surface resistance of copy paper decreases nearly six orders in magnitude. Dry paper has very good electric insulating behaviour so that thereon by corona discharge a fairly high electrostatic charge can be deposited before breakdown takes place. On using dry receptor paper the toner attraction force caused by said electrostatic charge can be built up with a reasonable corona charge. Since the leakage of charges through the receptor paper is a function of moisture content (paper humidity), a careful control of said moisture content will be in favour of toner transfer efficiency, image quality and reproducibility in toner printing results.
It is one object of the present invention to provide an electrostatographic printing apparatus with means for controlling the electrical condition of a paper receptor material whereby a higher yield of transferred toner is obtained and consequently less toner waste is formed, thereby reducing or avoiding quality deviations of transferred toner images and, in the case of double-sided (duplex) printing, improved performance may be achieved as a result of limiting the flow of electrostatic charges through the receptor material.
It is still another object of the present invention to provide a method for conditioning a moving paper receptor material to enable subsequent reproducible production of transferred dry toner images thereon.